1. Field of the Invention
This invention relates to an apparatus and method for measuring myocardial oxygen consumption. More particularly, the present invention relates to determining myocardial oxygen consumption by comparing oxygen saturation in atrial and mixed venous blood.
2. Description of the Background Art
Pulmonary artery catheters (“PACs”) are widely used for patient diagnosis and for hemodynamic and therapeutic monitoring. One of the most widely used PACs is the Swan-Ganz catheter. The Swan-Ganz catheter, a version of which is disclosed in U.S. Pat. No. 3,995,623 to Blake, includes a flexible tube (enclosing multiple lumina) that is designed to be flow-directed through a patient's heart by a distal balloon. The catheter is adapted to be delivered through the right atrium and right ventricle with the distal end positioned within the pulmonary artery.
The Swan-Ganz catheter includes first and second lumina for use in measuring blood pressures in the pulmonary artery and right atrium respectively. A third lumen is used for inflating the balloon at the distal end of the catheter. A fourth lumen is included for housing a thermistor that is used in monitoring blood temperature and in determining cardiac output. The fourth lumen also houses the wires associated with electrodes that are included for monitoring intraatrial and intraventricular electrograms. The Swan-Ganz catheter has been a useful tool in diagnosing complex cardiac arrhythmias.
A more recent PAC construction is disclosed in U.S. Pat. No. 6,532,378 to Saksena. In one embodiment, the PAC of Saksena includes a series of defibrillation electrodes interspersed with mapping electrode pairs at the distal end of the catheter. Proximal to the defibrillation and mapping electrodes are a series of sense electrodes and additional defibrillation electrodes. The catheter is used for indirect left atrial mapping from the left pulmonary artery and is also used in defibrillating or cardioverting the heart.
Each of the above referenced inventions is useful in providing a physician with information on the mechanical functioning of a patient's heart. However, none of the aforementioned PACs can be used to measure the rate of oxygen consumption by the heart, or myocardial VO2, whereby a physician may gain an understanding of the energy costs associated with the heart's performance. Measuring myocardial VO2 is significant because a decrease in myocardial VO2 may have serious consequences for critically ill patients. Heretofore, there has been no practical way to obtain myocardial VO2 measurements in critically ill patients.